Sea Ice, the Ocean's Fragile Cloak

Elizabeth Hunke
 

Will sea ice modelers soon be out of a job? After stunning sea ice losses last summer in the Arctic, one wonders! By the end of the 2007 summer melt season, the Arctic ice had shrunk by nearly 40 percent from its 1979-2000 average extent, and the fabled Northwest Passage was open to seafarers for the first time in human memory. Sea ice returns during the dark, cold, winter months, but will it recede to yet another record low next year? There are too many competing factors to know for certain, but the likelihood of an ice-free Arctic summer is rising
 
The Earth's atmosphere and ocean act as heat engines, always trying to restore a temperature balance by transporting heat away from the equator, toward the poles. Arctic sea ice, which covers a huge area---greater than the lower 48 states in summer and twice that in winter---regulates these circulation patterns and therefore our weather. It's disappearance could have significant implications for life on the planet.
 
Sea ice is simply frozen ocean water. It forms, grows, and melts in the ocean. In contrast, icebergs, glaciers, ice sheets, and ice shelves all originate on land. Sea ice occurs in both the Arctic and Antarctic, growing during the winter months and melting during the summer months, and some sea ice remains all year in both regions. But sea ice is like the snowmen that appear in neighborhood yards after a fresh snowfall---as soon as the temperature rises a tiny bit above freezing, they start to melt and soon are gone!
 
Ultimately, sunshine is king. It drives the climate system and melts the ice. Its disappearance at high latitudes in the winter allows the ice to grow back. But other factors are at work too, including the sea ice itself. Cold air from Siberia or the Antarctic continent cools the ocean's surface and new sea ice freezes. Winds blow it around, crashing it into the coast or icebergs or other sea ice, causing it to pile up into thick ridges of ice. Ocean currents bring warm waters beneath the ice, melting it from below. Sometimes winds and ocean currents together move the ice into warmer waters, where it melts.
 
In this tussle between atmosphere and ocean, the sea ice is not a passive bystander. It has its own tactics for meeting the competition or, in some cases, becoming an accomplice to its own destruction. Sea ice is both ocean sunscreen and blanket, preventing solar rays from warming the waters beneath and thwarting ocean heat from escaping to warm the air above. But if gradually warming temperatures melt sea ice over time, fewer bright surfaces are available to reflect sunlight, more heat escapes from the ocean to warm the atmosphere, and the ice melts further. The cycle accelerates. Thus, even a small increase in temperature can lead to greater warming over time, making the polar regions the most sensitive areas to climate change on Earth. But as sea ice melts, it leaves a layer of fresh water at the ocean's surface that inhibits the ocean's global circulation, the "conveyor" that brings warm water toward the poles. Sea ice is both an obstacle and a catalyst for change, able to hasten the pace in either direction.
 
What happens when there's no more Arctic ice in the summer? Polar bears lose their hunting platforms, for one thing. And they're at the top of the food chain! The web of life will be affected in ways we can not yet imagine, but the news may not be all bad: sea ice in the Antarctic retreats almost completely every summer, supporting a rich ecosystem at whose foundation lie algae and other microbes that thrive in the seasonal ice habitat. Will the Arctic become more like the Antarctic? Stay tuned: some fear the Arctic sea ice has already reached its tipping point.

Big Fast Shifts in the Ecology of New Mexico Have Begun Due to Climate Change

Craig Allen
 

Climate change is beginning to be evident in New Mexico, with markedly warmer and drier conditions expected in coming decades. These climate changes will stress existing forests, and likely drive increasingly extensive and severe episodes of forest dieback. This process seems to have already started. In summer 2002, pinyon (Pinusedulis) began dying en masse from drought stress and an associated bark beetle outbreak. Similer kinds of forest stress and dieback are now becoming apparent in many parts of the world. Warmer, dry conditions will also amplify the severity of fire activity, which can trigger massive erosion in mountain watersheds that could clog reservoirs that store water for human purposes. Water resources likely will be directly strained in New Mexico as projections of less winter snow means less free natural water storage in mountains watersheds and earlier spring runoff peaks, reducing water available in streams and reservoirs for human uses. Despite these trends, there are actions (like forest thinning) we can take to increase the resilience of forests in New Mexico to these expected effects of climate change.

The Burning Ice: Will Methane Hydrate Destabilization Surprise Climate Scientists?

Scott Elliott
 

The clathrates are an exotic substance formed just below the bottom of the sea, due to the reaction of methane decomposing from dead organisms with water molecules trapped in coastal sediment. If you bring methane clathrate crystals rapidly to the surface for study, or even just for your own amusement, they look and feel like common ice but can be set on fire. They are quite literally ice crystals that burn. No one really knows how much of the stuff is out there. A small but real chance exists that sea floor warming induced by global climate change will melt enough of the substance to inject significant quantities of free methane gas into the atmosphere. There it acts as a greenhouse agent and is thirty times more effective at trapping heat than carbon dioxide, which gets much more attention from scientists and policy makers. So what can be done to nail this problem down? We need to figure out where the clathrates are and then use global ocean models to simulate the rate at which they are likely to hiccup. After several years of trying, our local Los Alamos climate team has justrecently managed to get an okay to begin such a project.
 
Clathrate destabilization is in fact a good example of the many, greatly understudied but potentially significant climate phenomena that come under the general heading of "biogeochemistry". This may be defined as the study of everything happening near the surface of the planet that is not pure physics -in other words, all the interlocking biology, geology and chemistry of the ocean. atmosphere and continents. Biogeochemistry is a field of research just about to explode at the university and agency levels. Politicians and policy makers are beginning to realize that they cannot just study environmental change on an imaginary planet where there exists one element called carbon and one greenhouse gas constructed from it called CO2. Rather, they are compelled to consider the most complex chemical and biotic system known anywhere in the universe, the Earth, which is the place we happen to call home and are currently rebuilding in a major way. Biogeochemical feedbacks will have to be understood with relative completeness if we are to accurately predict the costs of altering and managing global climate.

An Introduction to the Science of Climate Change

Todd Ringler
 

Climate has always been a primary driver of civilization. Climate largely dictates what we wear, what we eat, what modes of transport we use and what type of dwellings we construct for shelter. Climate literally touches every facet of our lives. As such, climate has played a tremendous role in shaping the very fabric of our civilization.
 
Through millennia we have become accustomed to, and even comforted by, our relationship with the Earth's climate system. That relationship has always been a one-way relationship. The climate changes and we react to it. Even as we harnessed the power of nature by planting its river bottoms, damming its rivers, mining its ore and logging its forests we never fathomed the possibility that we could change the Earth's climate. How could we ever alter something so powerful and so immense as the climate by simply living our lives?
 
Through our own ingenuity and with gifts from nature, we have constructed our entire civilization on energy produced from oil, coal and natural gas. Unfortunately we are coming to realize that there are unintended consequences from our reliance on fossil fuels, mainly through the generation and emission of carbon dioxide. Once thought of as an innocuous gas, carbon dioxide is also a "greenhouse gas," meaning it traps heat within the atmosphere which, in turn, tends to warm the climate. So we are doing what was once considered unimaginable, we are changing the Earth's climate.
 
As we continue to study the Earth's climate system and how carbon dioxide is likely to alter it, we are continually amazed and humbled by the complexity and intricacy of the system. What starts as a small ripple in global temperature propagates into every part of the climate system where it can lead to consequences that far outsize the initial ripple. We are coming to appreciate the notion of "thresholds" in our climate system where a small changes acting over a long time, such as carbon dioxide emissions, leads to a large response that occurs over a very short time.
 
In this presentation I will try introduce the concept of global climate change along with the notion of thresholds. Examples of carbon-dioxide instigated thresholds in the climate system include rapid loss of Arctic sea ice, dramatic feedbacks driven by marine and terrestrial ecosystems and rapid sea-level rise due to melting ice sheets. I will touch on a few of these examples in an attempt to illustrate the two main points of the presentation. First, we have entered an era where we will determine the trajectory of the global climate system in ways we do not fully appreciate. And second, since the science of global climate change will continue to unfold for decades to come there are great opportunities for all of us, young and old.

What if the Ocean Conveyor Belt Stalled?

Wilbert Weijer
 

Imagine yourself drifting in the North Atlantic, attached to a bucket of water. You could be drifting in the middle of the ocean for months. Chances are that you will slowly drift towards the Caribbean, and enter one of the strongest currents on Earth, the Gulf Stream. Within a few weeks you will have been swept northward, past the coasts of Florida, Georgia, South Carolina and North Carolina. After passing Cape Hatteras, the current leaves the continent, and before you know it you will be in the middle of the Atlantic again. Now you have two choices. You can take the southern branch of the current, which will bring you straight across the Atlantic towards Africa. You might catch a glimpse of the Azores before you slowly drift south again towards the tropics. Or you can take the northerly branch. This will take you to Great Britain; from there you will drift north to Norway. Weather starts to deteriorate. The air gets colder, it starts to rain. Your call.
 
Suppose you took the northern branch. You might find yourself floating around in the Greenland Sea. Suddenly, your bucket of water will have cooled off so much that it becomes heavier than the water beneath you. It feels like the bottom drops out from under you, and you start to sink. You experienced a convection event, that will take you down to a depth of a few kilometers. Here, in the darkness of the abyss, you feel yourself slowly moving south again. You drift along the U.S. east coast again, but now in the opposite direction and several kilometers beneath the surface. You continue to drift, passing South America until suddenly you are swept eastward, you have entered the strong Antarctic Circumpolar Current. This majestic current flows around Antarctica, swept forward by the horrendous storms of the Southern Ocean. It might take you decades, and many trips around Antarctica, before you finally reach the surface again. If you happen to end up in the Atlantic Ocean, you will start to drift north again, first taking the Benguela Current to cross the South Atlantic from Africa to Brazil, then the North Brazil Current, across the Equator, until finally, hey! you are back in the Gulf Stream!
 
If you're still with me: congratulations! You just completed a loop of the global conveyor belt circulation! This ocean drift is very important for the climate system. When your water bucket cooled off in the Greenland Sea, it gave up its heat to the atmosphere, thereby heating the high northern latitudes, and in particular western Europe. There were times in the past when the water didn't get this far. During the ice ages, you would have sunk south of Iceland. Large parts of Europe, Asia and North America were covered by huge ice sheets, in part because the ocean couldn't get its heat as far north as it does today.
 
Climate scientists fear that there might be a hidden threshold in the conveyor belt. If the atmosphere heats up over the Greenland Sea, your bucket of water might not cool off enough to sink. In addition, chances are that the precipitation will increase as well. The extra freshwater in your bucket makes it even lighter in comparison to the cold and salty water underneath you. Without your bucket sinking, and drifting south, there is no place for new import of warm water. Global warming might thus slow down, and finally halt, the conveyor belt. If this happens, the impact will be felt mostly in the North Atlantic region; it will reduce or halt the trend of global warming. However, many consequences are hard to predict. What will happen to the sea ice in the Arctic? The ice sheets of Greenland? The North Atlantic fisheries that depend so strongly on the mild ocean temperatures? European agriculture? On the global scale, the impacts will be more subtle. The conveyor extracts a lot of heat and carbon dioxide (CO2) from the atmosphere. Without this sink, will CO2 levels and global temperatures go through the roof? As the deep ocean warms and expands, will the resulting raising sea levels mean the end of New Orleans, parts of Florida, the Netherlands?